The present invention relates generally to accurately determining the timing of the leading edge of a pulse of high frequency carrier signal, and, more specifically, to a method and apparatus for determining the leading edge timing of such a pulse at a receiver which receives the same pulse over multiple transmission paths of different lengths resulting in distortion of the pulse envelope.
In the field of ultrasonic position detection, pulses of high frequency carrier signal radiated by a signal source are received by several receivers spaced apart from one another. The distance and direction to the signal source may be computed from measured travel times of the pulses to the individual receivers. For accurate determination of the location of the signal source in such an arrangement, it is essential that each of the receivers recognize the same point in the received signal envelope as the beginning of the pulse for purposes of computing the travel times, and hence, direction and distance.
In most cases, a pulse will travel along multiple paths of different lengths between the signal source and each receiver. This results in a received pulse envelope which is stretched and distorted relative to the transmitted pulse, and which may not have a well defined or predictable leading edge. Further, it is not clear at which point in time during the leading edge of the envelope the pulse should be considered as having been received. Comparison of only signal amplitude with a predetermined threshold may not provide reliable criteria for determining pulse timing because the individual transmission paths will generally produce different signal attenuations, with attendant effects on the slope of the leading edge of the signal envelope. Further, the timing uncertainties are not avoided by adding a pulse shaping network which is triggered to initiate a square wave pulse solely in response to the received signal envelope reaching an amplitude threshold.
However, improvements in determining pulse timing can be realized by a variety of known approaches which combine amplitude threshholding with other detection criteria. For example, in one prior art approach, the slope of the leading edge of the received signal is evaluated by differentiating the signal envelope and detecting when the value of its derivative falls below a threshold comparison value. A valid pulse is determined to exist when one, and only one, negative crossing of the comparison value occurs during a time interval which commences when the envelope amplitude reaches a predetermined small value, and which has a duration not longer than the transient response period of the receiving transducer.
Although such an approach is capable of achieving detection which is relatively unaffected by many of the causes of pulse timing uncertainties, it requires somewhat complex signal processing operations and circuitry. Conversely, the applicants have provided a unique approach which achieves accurate and reliable determination of echo pulse timing with simple, low cost signal processing operations and circuitry.